Field of the Invention
The present invention relates to an image pickup apparatus, more particularly to an image pickup apparatus having the function of reducing the black sun phenomenon occurring particularly when shooting a highly-bright object. The present invention also relates to an image pickup system including such an image pickup apparatus and a lens apparatus.
Description of the Related Art
Many conventional image pickup apparatuses use a solid state image pickup element. CMOS image sensors are widely used as solid state image pickup elements. Some CMOS sensors are provided with a correlation double sampling (CDS) circuit. The CDS circuit generates a differential signal of a reset level signal (which will be referred to as N signal, hereinafter) and a signal picked up in a state in which light is incident on the image sensor (which will be referred to as S signal, hereinafter) and outputs the differential signal as a picked up signal. This process provides an improvement addressing the problem of fixed pattern noise generated in a solid state image pickup element by variations of characteristics among pixels and amplifiers provided for pixel rows.
However, there is a problem that when light having a high intensity greatly exceeding the signal saturation level is incident on the image sensor, leakage of light from a photodiode (PD) to a floating diffusion (FD) occurs, leading to an increase in the N signal level. Consequently, the difference between the S signal and the N signal becomes small in the CDS circuit, and the level of the output image pickup signal lowers. This phenomenon is called “black sun phenomenon”.
Japanese Patent Application Laid-Open No. 2009-200660 teaches to provide a clip circuit so as to prevent the aforementioned N signal level from exceeding a predetermined voltage value when high-intensity light is incident on the image sensor. According to Japanese Patent Application Laid-Open No. 2009-200660, even if light leaks to the FD upon the incidence of high-intensity light, the voltage level of the N signal will be limited by the clip voltage value set to the clip circuit. In consequence, the black sun phenomenon of the picked up signal generated from the CDS as the differential signal of the S signal and the N signal is mitigated.
Japanese Patent Application Laid-Open No. 2010-268162 discloses an improvement in which when black sun phenomenon occurs while picking up a highly-bright image, the output of the CDS circuit is replaced by a saturation output. In the technology disclosed in Japanese Patent Application Laid-Open No. 2010-268162, a pixel area in which the black sun phenomenon can occur is detected by prediction by applying different accumulation times to a first pixel group and a second pixel group.
In the aforementioned prior arts, it is difficult to picking up satisfactory images in a wide variety of shooting situations in both daytime and nighttime. For instance, in the case of the technology disclosed in Japanese Patent Application Laid-Open No. 2009-200660, if an excessive clip voltage is set, while the black sun phenomenon caused by high-intensity light can be eliminated or reduced effectively, fixed pattern noise is likely to be generated, leading to the problem of deterioration in image quality. Particularly in nighttime shooting, increasing a gain in order to increase the sensitivity can lead to a further deterioration in image quality, resulting in picked-up images with poor visibility. The technology disclosed in Japanese Patent Application Laid-Open No. 2010-268162 can reduce the black sun phenomenon on one hand, but is costly in manufacturing an image sensor having a structure that allows application of different accumulation times to different pixel areas on the other hand. Moreover, in the technology disclosed in Japanese Patent Application Laid-Open No. 2010-268162, a pixel group used as a reference for detection of black sun phenomenon is output by interpolation using pixels around it and not used as a picked-up signal. Consequently, the resolution of picked-up images is deteriorated. In the field of broadcast or advanced surveillance, various shooting conditions are imposed in daytime and nighttime, and high quality images are required to be picked up. Therefore, it is necessary to maintain high image quality while addressing the above-described problems.